Yes, I am amazed that Weiss felt the need to resort to using naturalized players...Guede is a capable footballer, but he does not have a strong link with Slovakia. Naturalization defeats the whole purpose of competitions between national teams (the coach is implicitly admitting that "his countrymen are not good enough" and this is of course not only unfair to the local lads who speak the language and are eager to represent their home country, but also shatters the confidence of the more established players). I cannot comprehend why so many sports federations are greedy for success (as you pointed out, Slovakia already made history by eliminating Italy and played well in their match vs the Netherlands) and prefer to embrace "the end justifies the means" philosophy.
The move backfired, as Slovakia suffered a shocking home loss against Armenia (0:4) and were quite disappointing in a number of other games as well. Eventually the Slovaks failed to reach the Euro 2012 finals. It's not a matter of individual mistakes/underwhelming performances (I am certain that Guede made sure to do his best for his adopted country, though that's not the point), but it seems as if the Slovak players lost faith in their coach and no longer felt very motivated. I have been reading a number of Slovak forums and the majority of supporters are not happy/find it more difficult to identify with the national team.
Naturalized players per national team (Euro 2012 finalists)
Croatia - 1: Eduardo da Silva (Brazilian)
Czech Republic - none
Denmark - none
England - none
France - none (however, a number of their footballers like Patrice Evra and Steve Mandanda were not born in France and were thus at one point eligible to represent other countries, e.g. Senegal and D.R. Congo)
Germany - 3: Claudemir Jerônimo Barreto - Cacau (Brazilian), Miroslav Klose and Lukas Podolski (they were born in Poland, but have both German and Polish ethnic roots and were raised in a German environment, so they could be considered borderline cases).
Greece - none
Italy - 3: Thiago Motta (Brazilian, but has Italian roots), Pablo Daniel Osvaldo (Argentine, but also has Italian heritage), Amauri Carvalho de Oliveira (Brazilian, possible Italian descent).
Netherlands - none at this stage, but Brazilian defender Douglas Franco Teixeira acquired a Dutch passport on the 2nd of November 2011 and the Dutch national team coach has stated that he could be called up to the national side. Some of the Dutch players like Vurnon Anita were born in the Netherlands Antilles or have Surinamese roots (Evander Sno), so were potentially eligible to represent other national squads.
Poland - 4: Roger Guerreiro (Brazilian), Damien Perquis (born in France, but has Polish heritage), Ludovic Obraniak (born in France, but from Polish descent), Eugen Polanski (born in Poland, has Polish roots, but has previously represented German youth teams). Nigerian Emmanuel Olisadebe is still an active footballer, but is unlikely to be a participant in the tournament.
Portugal - 5: Képler Laveran Lima Ferreira - Pepe (Brazilian), Rolando Jorge Pires da Fonseca (Cape Verde), LuÃs Carlos Almeida da Cunha - Nani (Cape Verde), José Bosingwa da Silva (D.R. Congo), Anderson LuÃs de Souza - Deco (Brazilian, possible Portuguese roots).
Republic of Ireland - 6 (possibly more): Jonathan Walters (English-born, Irish-born mother), Sean St Ledger (English-born, Irish heritage), James McCarthy (Scottish-born, family links to Ireland), Leon Best (English-born, family connection to Ireland), Paul Green (English-born, eligible due to family status within Ireland), Caleb Folan (English-born, family members' connection). It has to be acknowledged that these are not necessarily clearcut cases of opportunism, as most of the players in question do have Irish roots.
Russia - none
Sweden - none
Spain - none, Brazilian Marcos Senna is still an active footballer, but is unlikely to be called up.
Ukraine - 3: Artem Milevskiy (ethnic Belarusian, born in Belarus, has represented the Belarus U16 side), Marko Devich (ethnic Serb, born in Belgrade), Edmar Halovskyi de Lacerda (Brazilian). Some of the Shakhtar Donetsk Brazilians might also be eligible, but are unlikely to receive a call-up.